1. Field of the Invention
The invention relates to the technical field of a controller for a steering transmission ratio variable device that controls a steering transmission ratio variable device represented by variable gear ratio steering (VGRS).
2. Description of the Related Art
Some steering transmission ratio variable devices are provided with a locking mechanism for restricting relative rotation of a steering input shaft and steering output shaft, and an example thereof is disclosed in Japanese Patent Application Publication No. 2008-068782 (JP-A-2008-068782). According to the steering transmission ratio variable device disclosed in JP-A-2008-068782, a driver can be prevented from being aware of a locking sound of the locking mechanism by generating an end contact sound originating in a mechanical stopper roughly simultaneous to the locking sound of the locking mechanism based on steering angular velocity and threshold steering angular velocity at the locking.
Furthermore, in the case VGRS is in a locked state, determination of a current limit value corresponding to the magnitude of an output torque has been proposed as described in, for example, Japanese Patent Application Publication No. 2009-166598 (JP-A-2009-166598).
According to the technical idea disclosed in JP-A-2008-068782, the timing at which locking of the locking mechanism is completed is roughly equal to the timing at which the turning angle of wheels to be turned reaches a turning angle end position (namely, end contact timing) determined based on the physical configuration of the steering mechanism.
Here, since the steering mechanism has high physical rigidity at the turning angle end position, in the case, for example, a driver has operated a steering wheel or other steering input device in a direction of increased turning at the turning angle end position or within a turning range in the vicinity thereof, steering torque as steering load increases suddenly.
On the other hand, although a steering transmission ratio variable device employs a configuration in which a steering input shaft and a steering output shaft are rotated relative to each other by an actuator that includes a motor, due to the nature of relative rotation, the motor is rotated in the case steering torque has been applied to the steering input shaft even though the steering output shaft is unable to rotate further at the turning angle end position.
At that time, since steering torque increases suddenly as previously described within a turning angle range in the vicinity of the turning angle end position, the motor rotating speed transiently and easily increases excessively. In particular, in a configuration in which motor rotating speed is transmitted to the steering output shaft after suitably reducing with a speed reduction mechanism, steering torque is transmitted to the motor accompanying an increase in speed corresponding to the gear ratio of the speed reduction mechanism.
Thus, in the device configuration disclosed in JP-A-2008-068782, a motor has the potential to excessively increase speed when a steering transmission ratio variable device is switched by a locking mechanism to a locked state in which the relative rotation is restricted. Since there are many cases in which locking mechanisms typically realize a locked state due to an engaging action of corresponding engaging members, in a device configuration of this type in which switching is carried out to a locked state at a turning angle end position or within a turning range in the vicinity of a turning angle end position, there is the potential for the load applied to the locking mechanism to become large when switching from an unlocked state to a locked state.
In addition, in a conventional device configuration that includes the device disclosed in JP-A-2008-068782, the motor frequently only supplies a so-called holding torque when switching to the locked state without considering changes in steering load when switching to the locked state. Thus, in the case steering load has changed excessively in the direction of increasing or decreasing during the switching period to the locked state without necessarily limiting to the turning angle end position or a turning angle range in the vicinity of the turning angle end position, the balance between steering load and holding torque is disrupted and the motor increases speed, thereby resulting in the possibility of a similar increase in the load applied to the locking mechanism.
On the other hand, problems occur in terms of costs and installability when an attempt is made to secure reliability for this large load.
In this manner, the technical idea disclosed in JP-A-2008-068782 has the technical shortcoming of being susceptible to an increase in the load applied to the locking mechanism. This applies similarly to the device disclosed in JP-A-2009-166598.